import React, { useEffect, useState } from 'react'

export default function App1() {
  let [opacity, setOpacity] = useState(1)
  useEffect(() => {
     //初始化的时候添加一个定时器
    const opacityTimer= setInterval(() => {
      opacity -= 0.1
      if (opacity < 0) {
        opacity = 1
      }
      setOpacity(opacity)
    }, 100)
    return ()=>{
      clearInterval(opacityTimer)
    }
  },[])
  return (
    <div>
      <h1 style={{ opacity: opacity }}>react真难，不学了</h1>
    </div>
  )
}
